The present invention relates to a device for optimising distribution of a fluid comprising at least one gas phase at least a portion of which is usually formed by hydrogen and at least one liquid phase, for example, through a bed of granular solid or particles, the fluids circulating in a substantially downflow mode through said bed of granular solid (or solid particles). The invention also relates to a vessel comprising an inlet for a first liquid fluid and for a second gaseous fluid close to its upper end, containing at least one bed of granular solid and with a device of the present invention as will be described below located above said bed.
This device can be disposed.
either at the head of the vessel, which usually is a reactor,
or at the outlet from a granular bed (supplying the subsequent granular bed over the whole cross section of the vessel)
The present invention is of particular application in all cases:
where the gas phase is in the vast majority compared with the liquid phase, i.e, where the ratio between the gas and the liquid is usually more than 3:1 by volume and normally less than 400:1 (3 less than gas vol/liquid vol less than 400);
where the reaction is highly exothermic and necessitates introducing a supplemental fluid, usually a gas, into the reactor to cool the gas/liquid mixture where the reaction requires intimate contact to allow a compound (for example hydrogen H2) to dissolve in the liquid phase.
In particular, the present invention is applicable to gas/liquid distributors such as those used to carry out hydrocracking, hydrotreatment, hydrodesulphurisation, hydrodenitrogenation, total or selective hydrogenation of C2 to C5 cuts, selective hydrogenation of steam cracking gasoline, hydrogenation of the aromatic compounds in aliphatic and/or naphthenic cuts, and hydrogenation of olefins in aromatic cuts.
It is also applicable to other reactions requiring good mixing of a gas phase and a liquid phase, for example partial or complete oxidation reactions, or amination, acetyloxidation, ammoxidation or halogenation reactions, in particular chlorination.
In the specific field of hydrodesulphurisation, hydrodenitrogenation and hydrocracking, to achieve high efficiency conversions (to obtain a product containing, for example, 30 ppm (parts per million) of sulphur or less), a good distribution of gas and liquid, principally liquid, is necessary as the volume ratios are generally between about 3:1 and about 400:1 and usually about 10:1 to about 200:1, in the case of quenching, very good contact is required between the gas introduced to carry out cooling and the fluids from the process being carried out, usually termed the process fluids.
Because of the small proportion of liquid compared with the gas, one possibility used in the prior art consists, for example, of using distributor trays comprising a plurality of apertures for the passage of liquid and a plurality of downcomers for the passage of gas Descriptions of such devices can be obtained, for example, from U.S. Pat. No. 3,353,924, U.S. Pat. No. 4,385,033 and U.S. Pat. No. 3,855,068.
However, such solutions cause problems as regards the flexibility of use of the trays, and can also result in irregular supply from the different orifices if the trays are not perfectly horizontal and/or the because of the backflow caused by the huge drop in liquid and gas streams on the trays.
To overcome such disadvantages, the skilled person has been directed to use a specific arrangement of a plurality of trays the last one being either provided with means for collecting and distributing the liquid and gas phases in a separate manner as described, for example, in U.S. Pat. No. 5,232,283, or in the form of a mixture as described, for example, in U.S. Pat. No. 4,126,539, U.S. Pat. No. 4,126,540, U.S. Pat. No. 4,836,989 and U.S. Pat. No. 5,462,719.
The major disadvantage of such systems is that because of the small quantity of liquid with respect to the gas, in order to attempt to sprinkle the whole surface of said bed of granular solid properly, the skilled person is led to use a high density of downcomers, usually more than 80 downcomers per square meter as mentioned in FR-A-2 745 202. The gas velocity in the downcomers is generally from 0.5 to 5 centimeters per second (cm/s) and the liquid velocity is generally 0.05 to 1 cm/s. These velocities are, however, too low to allow simultaneous mixing and dispersion.
Because of this absence of liquid dispersion at the outlets from the downcomers, the skilled person is often constrained to install deflector plate type systems at the outlet from the orifices or downcomers as described, for example, in French patent FR-A-2 654 952, International patent application WO-A-97/46303 and in U.S. Pat. No. 5,799,877. All jet disturber type systems described in the prior art are associated with an aperture and/or a downcomer and they are shaped either as a solid impact plate as described in U.S. Pat. No. 5,799,877, FR-A-2 654 952 and U.S. Pat. No. 4,160,625 downstream a venturi shape downcomer, or are a receptacle with very low walls as described in WO-A-97146303. The disadvantages of that type of system arise from the fact that the jet disturber device does not cover the entire surface area of the reactor and that the portion of the granular solids located below said jet disturber system has very little chance of being sprinkled with liquid.
The prior art can be illustrated by the patent U.S. Pat. No. 3,524,731 and U.S. Pat. No. 3,431,084 and by the patent U.S. Pat. No. 3,824,080 that describes a device for mixing a gaseous phase and a liquid phase displaying a collector plate for the liquid phase that induces a converging movement toward a central mixing zone, inside which the liquid phase collides with the gaseous phase. None of these patents provide a teaching or a suggestion of a jet disturber device allowing a complete utilisation of the solid particles bed.
It has now been discovered that it is possible to obtain more effective sprinkling and thus better use of a bed of granular solid by reducing the density of downcomers or mixer conduits per square meter and by providing the distributor tray and/or the downcomers or mixer conduits of a porous dispersive system or an jet disturber device with a controlled porosity allowing the liquid jet at the downcomer outlet to be dispersed.
In the device of the invention, the number of mixer conduits is selected so as to increase the velocity of the different phases constituting a mixture and thus to obtain a sufficient energy to render the dispersive system effective, to provide better contact of the resulting mixture with the catalytic bed, and better homogeneity in the flow rate of the poly-phase mixture.
Advantageously, by selecting the distance separating the distributor tray and the dispersive system and the distance between the dispersive system and the bed of granular solid, this distribution can be further increased by preventing separation or sub-division of the mixture formed inside the downcomers when it traverses the distance separating the distributor tray and the dispersive system and when it transverses the distance separating the dispersive system and the catalytic bed.
The present invention concerns a device for bringing into contact and distributing a mixture comprising at least one gas phase and at least one liquid phase for a vessel containing at least one bed of granular solid, said phases being in overall downflow mode through said bed of granular solid.
In its broadest aspect, the device of the invention is defined as a device for distributing a poly-phase mixture comprising at least one gas phase and at least one liquid phase, said mixture being in downflow mode through at least one bed of granular solid, comprising:
at least one tray (P) located above one of said beds of granular solid;
a plurality of mixer conduits (21) for said liquid and gas phases of said mixture, each of said conduits comprising at least one upper cross section for flow (22) and at least one lower cross section for flow (23) allowing the mixture formed inside said mixer conduits to communicate with a bed of granular solid, said mixer conduits being provided with one or more lateral cross sections for flow (26) over a portion of their height;
said upper cross section for flow (22) allowing the majority of the gas phase of said mixture to pass and said lateral cross sections for flow (26) allowing the passage of the liquid phase into said mixer conduits and/or at least a portion of the gas phase respectively, said device comprising at least one jet disturber type dispersive system (28) with a controlled porosity located below the lower cross section for flow (23) and above the bed of solid.
In a particular implementation, the jet disturber type dispersive system (28) is characterized in that the void surface area with respect to the total surface area (porosity) is in a ratio from about 2% to about 80%, preferably about 5% to about 50% and usually about 5% to about 30%. The porosity range of the jet disturber device will be chosen according to the superficial velocity of the gas and the liquid phase, to the specific gravity of the gas and the liquid phase, to the viscosity of the gas and the liquid phase and according to the surface tension of the liquid in relationship with the nature of the surface of the jet disturber device.
In a particular embodiment, the device of the invention is a device in which a jet disturber type dispersive system is associated with each mixer conduit.
In a further particular embodiment, the device of the invention is a device in which a jet disturber type dispersive system is associated with a plurality of neighbouring mixer conduits.
In a further particular embodiment, the device of the invention is a device in which a jet disturber type dispersive system is associated with all of the mixer conduits of the device.
In a particular embodiment, the jet disturber type dispersive device of the invention is associated with at least one element for connection to at least one tray (P) or to at least one support beam for said tray (P).
In a further particle embodiment, the jet disturber type dispersive device could be located at the same level. They can also, in another further particular embodiment, be located at least at two different levels, in such a way that, preferably, the projection on the cross section of the reactor of these dispersive device belonging to different levels does not lead to superposed zones and leads to an approximately complete covering of the entire cross section of the reactor.
The distance between two successive levels is generally in the range of 1-250 mm, preferably of 5-180 mm and usually of 10-80 mm.
The distances between two successive levels are generally the same. This arrangement of the jet disturber type dispersive devices allows a better flow pattern of the gas phase, especially in case of fluctuating flow rate of the gas phase leading to some momentary extra flow rates of said gas phase.
In a further particular embodiment, the jet disturber type dispersive device of the invention, could be located at different distances from each other with respect to at least one tray (P) or to at least one support beam of said tray (P) to which they are connected by at least one connection element.
In a further particular embodiment of the jet disturber type dispersive device of the invention, at least two jet disturber type dispersive devices located at different distances are connected together by at least one connection element and the dispersive system or systems located at the shortest distance from at least one tray (P) or at least one support beam for said tray (P) is connected to said tray (P) or said support beam by at least one connection element.
In the device of the invention, the density of the mixer conduits is usually low, for example about 1 to 80 conduits per square meter, preferably 5 to 50 conduits per square meter.
The mixer conduits with a tabular shape, have generally an approximatively constant cross section. They preferably have a cylindrical pattern.
The mixer conduit generally has a diameter in the range of 0.3-10 cm, preferably 0.3-5 cm and usually, for example, in the range of 1-5 cm.
The tray can also comprise orifices for draining the liquid phase. In this case, the total surface area resulting from the sum of the flow surface areas of each of the orifices is such that the flow rate of the liquid phase through the drainage orifices is less than 10% of that of the liquid phase fraction during operation, preferably less than 5%.
The device of the present invention has the following advantages over prior art devices:
by reducing the number of mixer conduits, the gas and liquid velocities in the mixture inside the mixer conduit are increased and thus mixing and contact between the phases is encouraged;
by adding a dispersive system covering almost the whole of the reactor, optimum use of the bed of granular solid is ensured;
by using a small number of mixer conduits, a self-supported tray can be used and thus the volume of the device is minimised and the obstacles encountered by the fluid are minimised;
the device is simple and robust (a single tray in contrast to prior art devices, as described, for example, in U.S. Pat. No. 4,126,540).